1. Field of the Invention
The present invention relates to charged particle accelerators, and more particularly, to a cyclotron resonance accelerator having multiple cavity stages with a uniform magnetic field across each stage in order to provide substantially increased efficiency.
2. Description of Related Art
There are several applications for charged particle accelerators that will produce particles with energies equal to about two or three times their rest mass energy. For example, electrons (rest mass equivalent to 511,000 volts) when accelerated with 1 million volts produce X-rays which have the right energy for determining the density of rock, a property important in determining whether or not the rock is porous enough to contain oil. One to several million electron volts is also the right energy for X-rays used in food sterilization to insure against e. coli, salmonella, and listeria contamination. Protons (rest mass equivalent to 943,000,000 volts) when accelerated to about one billion volts have a large cross section for the production of neutrons when they collide with the nuclei of heavy metals such as lead, mercury, or tungsten. These neutrons are capable of driving sub-critical reactors. Such sub-critical reactors use fissile nuclear fuel more efficiently, consume long-lived actinides and hence reduce the geologic storage problem relative to that of waste from conventional nuclear reactors. In none of these accelerator applications is it important that the beam of particles be focused on a small spot as is the case for imaging X-ray tubes. In these applications a diffuse impact zone is an advantage because it helps solve an otherwise difficult thermal problem.
In high-energy machines, linear acceleration is useful because it eliminates losses due to synchrotron radiation. In high-current machines, linear accelerators are useful because the loading of the beam on each cavity can be large compared to the losses in the cavity due to electrical resistance of the cavity material. This is particularly true for pulsed machines in which cavity losses are minimized by turning off the RF power between high-current beam pulses. In continuous-current machines, in which a requirement for a low-emittance, well-focused beam exists, the beam loading is so small that super-conducting cavities have had to be used to solve the cavity loss problem. Otherwise, circular machines in which the beam orbits in the same cavity many times are much more efficient because the beam loading is increased, relative to the losses, roughly in proportion to the number of times the beam passes through the cavity. The problem with circular machines is that the cyclotron frequency changes as the relativistic mass of the particle changes with energy. In general, a particle is accelerated as long as the frequency of the accelerating voltage is below the relativistic cyclotron frequency of the particle in the magnetic field. As the particle gains energy, the relativistic cyclotron frequency falls below the frequency of the xe2x80x9cacceleratingxe2x80x9d voltage and the particle gives some of its energy back to the xe2x80x9cacceleratingxe2x80x9d electric field.
In 1945, Veksler in the U.S.S.R. and McMillan in this country pointed out that relativistic particles tend toxe2x80x9cbunchxe2x80x9d and remain stable with respect to the phase of the accelerating voltage. Thus, the limitation on energy imposed by the change in cyclotron frequency with energy in a conventional cyclotron can be dealt with by changing either the frequency of the accelerating voltage or the magnetic field as is done in the synchro-cyclotron or the synchrotron, respectively. If these changes are made slowly enough, charged particles gain energy as the frequency is lowered or the magnetic field is raised. Such beams are not continuous, but instead are extracted from the device after the desired energy level has been reached.
In 1958 and 1959, Twiss, Gaponov, and Schneider recognized that electrons traveling along helical paths in a transverse RF electric field and a steady axial magnetic field could be bunched azimuthally through the mechanism of the relativistic mass change. They could also radiate at a frequency near the cyclotron frequency. This interaction is now sometimes called the xe2x80x9ccyclotron resonance maserxe2x80x9d (CRM) instability. Hirshfield and Wachtel at Yale both observed the CRM instability and calculated its characteristics. It is probably correct to think of the CRM instability as the inverse of synchrotron acceleration with the addition of axial motion to the electrons. Jory and Trivelpiece accelerated electrons with 1,000 volts of energy traveling along the axis of a TE111 circular waveguide cavity to 500,000 volts of energy with momentum directed primarily in the circumferential direction. They used these electrons to generate millimeter wavelength radiation in another circular waveguide supporting a higher order mode.
More recently, Hirshfield has built more sophisticated inverse CRM accelerators. He built an electron accelerator similar to the device described above except that the magnetic field increased along the axis of a waveguide supporting a TE11, mode so that the Doppler shifted RF electric field maintained synchronism with the relativistic cyclotron frequency. This kind of device is called a Cyclotron Auto-Resonance Accelerator (CARA). Hirshfield developed the computer codes necessary to simulate the motion of charged particles in static magnetic and high-frequency electromagnetic fields. Hirshfield first tried a CARA for electrons. The results showed that an energy equal to twice the rest mass energy could be reached with achievable field strengths, but the efficiency was not impressive. Simulations for protons were very disappointing. The proton particles made very few orbits in the magnetic field before mirroring occurred. Because the axial magnetic field in a CARA increases with axial distance, there must be a radial magnetic field. This interacts with the angular velocity of the particles, eventually stops the beam, and sends it back along the axis. For the CARA for protons, it turned out that unless the electric field in the cavity and the consequent losses are very high, the protons stopped before making enough orbits to gain anything close to the desired energy.
Accordingly, it would be advantageous to provide an accelerator capable of accelerating a particle to an energy equal to at least twice its rest mass with high efficiency, without the stalling problem of known cyclotron auto-resonance accelerators.
In accordance with the teachings of the present invention, a high-current, high-gradient, high-efficiency, multi-stage cavity cyclotron resonance accelerator (MCCRA) provides energy gains of over 50 MeV/stage, at an acceleration gradient that exceeds 20 MeV/m, in room temperature cavities.
The multi-stage cavity cyclotron resonance accelerator includes a charged particle source, a plurality of end-to-end rotating mode room-temperature cavities, and a solenoid coil. The solenoid coil encompasses the cavities and provides a substantially uniform magnetic field that threads through the cavities. Specifically, the MCCRA is provided with a constant magnetic field sufficient to produce a cyclotron frequency a little higher than the RF of the accelerating electric field. A plurality of input feeds, each of which are respectively coupled to a cavity, are also provided. According to an embodiment of the invention, the beam from the first cavity passes through a cutoff drift tube and is accelerated further with a cavity supporting a still lower radio-frequency electric field. This embodiment yields a several-milliampere one-gigavolt proton beam efficiently. The single cavity transfers about 70% of the radio-frequency energy to the beam. A multiple-cavity accelerator using a constant or slightly decreasing static magnetic field along its length and using cutoff drift tubes between the cavities operating at progressively lower frequencies, each somewhat lower than the local relativistic cyclotron frequency of the beam in that cavity, provides an extremely-efficient, compact, continuously-operating, medium-energy accelerator.
The magnetic field in the accelerator is substantially uniform across all stages, since an increasing field would lead to undesirable loss of axial momentum and stalling, while a decreasing field would lead to an unmanageable increase in orbit radius. Successive cavity stages of the accelerator will operate at successively-lower RF frequencies to maintain approximate resonance as the particle mass increases.
In an alternative embodiment, the cavity diameters are reduced by using dielectric loading in the form of a thick coaxial dielectric liner. In yet another alternative embodiment, thick radial vanes are employed in the cavity that provide capacitive loading and thereby reduce the cutoff frequency for the desired dipole modes. When four symmetric vanes are used, the structure resembles that for a radio-frequency quadrupole (RFQ), except that it is the two degenerate dipole modes that are of interest rather than the quadrupole modes. To obtain a rotating (i.e., circularly polarized) field, these two dipole modes are excited in time-quadrature. The structure can be labeled a radio-frequency double-dipole (RFDD).